This invention relates to provision and use of polyolefin, notably poly-xcex1-olefin, particularly polypropylene, having remarkably high melt flow rates, provided in pellet form, and articles made from such material.
It is known that poly-xcex1-olefins, particularly polypropylene, may have its weight average molecular weight decreased substantially, or its melt flow rate substantially increased, by controlled degradation of the polymer. This may be accomplished by any of several means including reaction of the polymer with free radicals or free radical-producing agents, heat treatment, subjection of the polymer to high shear, or high strain in a manner to generate heat, trigger peroxide reaction with the base polyolefin, other means, or combinations of these. The effect attained is that polymer molecule scission occurs resulting in an overall lowered molecular weight or elevated MFR. Addition and reaction of peroxide with polymer is well known in the industry and is known generally as vis-breaking or peroxide degradation.
U.S. Pat. No. 4,665,118 issued to Bourland May 12 1987 describes a vinyl molding compound having high melt flow rate propylene-based polymer and calcium stearate incorporated which exhibits reduced processing energy upon molding.
U.S. Pat. No. 4,971,937 issued to Albizzati et al. Nov. 20, 1990 describes solid catalyst components for the polymerization of olefins and modified with electron-donor compounds, comprising a titanium halide supported on a magnesium dihalide in active form and containing as an electron-donor compound.
U.S. Pat. No. 4,978,648 issued to Barbe et al. Dec. 18, 1990 describes catalysts for polymerization of olefins.
U.S. Pat. No. 5,427,845 issued to Sawyer et al. Jun. 27, 1995 describes Melt-spun filaments having a highly crimped configuration which is imparted by differential cooling, non-woven webs of the crimped melt-spun filaments, and a process of forming a nonwoven web of the crimped filaments. Such filaments are formed from a random copolymer of propylene and ethylene or an xcex1-olefin of at least four carbon atoms which provides an enhanced response to crimping by differential cooling.
U.S. Pat. No. 5,476,911 and 5,529,850 issued to Morini et al. describe crystalline propylene polymers having MWD less than or equal to about 3.7 and with MFR in the range of 600 to 1,000 dg/min and Mw in the range of 100,000 to 60,000, and Mz greater than or equal to 140,000 which are obtained by way of polymerization with stereospecific supported catalysts. The resulting polymer is then subjected to degradation by free radical generators, particularly organic peroxides. Fibers produced from such propylene polymers are also described.
EP 0 362 705 A2 describes catalysts, for the polymerization of xcex1-olefins, being the reaction product of an aluminum alkyl, an ether containing at least two ether groups which can form complexes with anhydrous Mg Cl2 and a solid catalyst component comprising anhydrous magnesium dichloride in active with a supported titanium compound of particular electronic configuration and an electron-donor compound.
EP 0 361 494 A2 describes components and catalysts for the polymerization of olefins being solid and modified with electron-donor compounds, comprising titanium halide supported on magnesium dihalide in active form and containing as an electron-donor compound a di- or polyether having specific reactivity characteristics toward MgCl2 and TiCl4.
EP 0 480 190 A2 describes propylene and other xcex1-olefin polymers having specified crystallinity and isotacticity with apparently low xylene solubilities and molecular weights between 5,000 and 100,000.
EP 0 320 150 A2 describes an ultra-high molecular melt flow rate propylene polymer in which the molecular weight distribution in not inversely proportional to the melt flow rate which is produced without post-reactor treatment by contacting propylene and hydrogen in the gas phase in the presence of a high activity catalyst.
DE 27 20 701 describes polyolefins of differing tacticity, crystallinity with and without substituents for pendant hydrogens. Some are copolymers, with some of these being block copolymers.
Morman et al., issued May 29, 1984, describes thermoplastic polymers with improved processability resulting from initial partial degradation of high molecular weight polymers using a chemical prodegradant present in excess of the amount of the amount reacted during pelletization. Polypropylene having high molecular weight, in the range of from about 250,000 to 1,000,000, is noted as being the typical range for commercially produced polypropylene; it is further noted that polypropylene with weight average molecular weight less than about 130,000 cannot be easily pelletized and that, therefore, polymer producers will prefer delivering polypropylene having weight average molecular weight greater than about 160,000.
I have found that use of lower initial molecular weight polymer, or higher melt flow rate polymer, will generate less frictional heat in compounding into pellets. This means that viscous dissipation of the heat will cause less peroxide activation and allow pelletizing at generally lower temperatures or at longer exposure periods. Using lower weight average molecular weight (Mw), or higher melt flow rate, starting material requires less of the vis-breaking agent, such as peroxide, to reach the desired very high or ultra-high melt flow rates. Therefore, the material will be less prone to difficulties with unreacted, or non-degraded, high molecular weight polymer in the form of gels. Such gels, upon formation of non-woven fabric lead to a type of defect called xe2x80x9cshotxe2x80x9d in the fabric, or a pass-through plug of hard material. Further benefit may be found in the formation of less of the undesirable by-products of peroxide degradation.
Practice of my invention allows use of polyolefins, particularly polypropylene, having a starting weight average molecular weight in the range of up to about 120,000, compounding with a chemical degradant, pelletizing, and delivery of pelletized polymer having a final effective melt flow rate of greater than about 600 dg/minute.
Our invention provides, at least, polyolefin, notably poly-xcex1-olefin polymer, particularly polypropylene, having ultra or extremely high melt flow rates. Such flow rates will be in the range of at least about 600 g/10 minutes through at least about 2,000 g/10 minutes. Such polymer may be homopolymer or copolymer and is produced in and supplied as pelletized polymer rather than as the granular reactor-grade product which is typically provided in applications in which high melt flow rate polymers are desired. Products formed from this high melt flow rate, or low molecular weight material constitute part of this invention.